The present invention relates to switching power supplies which use an inductor, and more particularly to a merged boost and polarity inverting switching power supplies in hard disk drives.
The positive boost switching power supplies typically includes an inductor which has the supply end connected to a power source, with the output end of the inductor connected to a driver and the anode end of a diode (or series of diodes). The cathode end of the diode is connected to the positive output storage capacitor. In the storage portion of the cycle, the driver pulls the output end of the inductor to near ground to store energy in its magnetic field. In the boost portion of the cycle, the driver turns off, the inductor voltage flies high, and the inductor""s stored energy is transferred through the diode to the positive output storage capacitor. When the driver senses the desired output voltage has been reached on the positive output storage capacitor, the driver may reduce the storage portion of the cycle or may skip the storage portion of the cycle until the output voltage drops below the desired regulated voltage.
When a polarity inverting negative switching power supply is typically merged with the positive boost switching power supply as described above, a transfer capacitor is also connected to the output end of the inductor. The other end of the transfer capacitor is connected to the anode end of a diode to ground and the cathode end of the diode whose anode is connected to the negative output storage capacitor. In the storage portion of the cycle, the driver pulls low to transfer charge from the transfer capacitor through the diode to the negative output storage capacitor. In the boost portion of the cycle, the driver turns off, the inductor voltage flies high, and the inductor charges the transfer capacitor through the diode to ground.
The positive output voltage can be regulated to any voltage more positive than the input supply voltage. Since only one output can be regulated in a merged boost switching power supply, the negative output voltage will not be well regulated and is somewhat dependent the output loads and on the number of diodes used in series with the capacitors. This application of the positive and negative boost switching power supply regulates to 25 volts Vpp (positive voltage) output, and approximately 24 v Vnn (negative voltage) output when one diode is used between the inductor and the positive storage capacitor. This application used a 2 MHz constant clock frequency. This driver application uses a NFET to pull the inductor output down to ground, is current limited to approximately 100 mA, and the driver is turned off when the current limit is reached (to reduce NFET power dissipation and increase efficiency). In this application, when Vpp exceeds it""s regulated voltage, the driver skips the storage portion of the cycle to avoid overcharging, until the output voltage drops below the regulated voltage.
One problem with a switched power supply is when most of the inductor""s stored energy has been transferred to the Vpp positive output capacitor; the inductor""s voltage will fall lower than the positive output capacitor voltage plus a diode. At this point, the inductor load changes to a high impedance and the inductor rings from a positive peak voltage near the positive output capacitor voltage and a negative peak voltage near ground at a frequency much higher than the clock frequency. This high frequency may be coupled to other circuits to effect their operation.
The present invention includes a shunt circuit between the inductor output and the input supply, and as a consequence, the inductor voltage is shunted at times in the cycle, quickly reducing the ringing of the inductor voltage.